Revenue from Contracts with Customers Revenue from Contracts with Customers (Balance Sheet Impact of the Adoption of ASC606) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Item Effected [Line Items]
Prepaid expenses and other	$ 7,724		$ 6,233
Other assets	27,470	$ 27,028	16,230
Current portion of other long-term liabilities	14,587	15,750	13,002
Other long-term liabilities	104,286	109,940	100,203
Deferred income taxes	136,600	133,064	133,477
Retained earnings	805,993	798,128	$ 799,402
Mid-Stream [Member] | Adjustments due to ASC606 [Member]
Item Effected [Line Items]
Prepaid expenses and other	50
Other assets	11,397	10,798
Current portion of other long-term liabilities	2,824	2,748
Other long-term liabilities	9,118	9,737
Deferred income taxes	(121)	(413)
Retained earnings	(374)
Adjustment to opening retained earnings, after tax		$ (1,274)
Mid-Stream [Member] | Amounts without the Adoption of ASC606 [Member]
Item Effected [Line Items]
Prepaid expenses and other	7,674
Other assets	16,073
Current portion of other long-term liabilities	11,763
Other long-term liabilities	95,168
Deferred income taxes	136,479
Retained earnings	$ 805,619